I can also hear a difference when the bobbin is out

I have found that my warning for low bobbin thread does not work when my bobbin area needs a good cleaning. Try taking the bobbin area apart and giving it a good cleaning before taking it in for servicing. Hope that does it for you.

I'd say clean the bobbin area and give it a try. I have used canned air since a teacher at my dealer showed how to use it by first removing the throat plate and holding the can behind the machine. That way the air blows out the front. I do also have a small vacuum cleaner that I used between projects. Usually, though, just cleaning with the tiny brush between bobbin fill-ups works find. Of course, I don't have a warning signal, so I can really address your problem.
Wishing you good luck solving the problem!

I actually noticed this problem right AFTER I cleaned out the machine. I've been changing bobbin colors for different projects recently so a bobbin hasn't had the opportunity to get empty. Maybe I pushed a piece of lint someplace that it shouldn't be. I'll go back and clean it again. Thanks everyone!!!

On my machine, the sensor which triggers the ringer doesn't work if the bobbin area is clogged up with dust and lint.
Try cleaning your bobbin area and this might make a difference.

My machine seems to do that every now and then, Sometimes cleaning the bobbin case thoroughly helps as others have said. I'm not sure I would take my machine in just for this unless it was due for a tuneup. I only have one machine and I am lost without it.

Sometimes there is a little bit of lint in the bobbin area that blocks the bobbin sensor. It can be just a tiny speck. Clean the bobbin area with a lint brush and see if that helps.

I don't have a warning bell but I can hear a distinct difference in the machine when the bobbin empties

When my bobbin bell doesn't go off it means I need to clean out my bobbin area.

This happened to me even though I cleaned the area out and tried different bobbins. I just learned to live with it. A few months later when I had the machine in for its annual tune-up I mentioned it to the technician. He checked it out and found that the little light bulb that sends a beam of light through the plastic bobbin was burned out. He replaced the part, it worked and I was a happy camper.